1. Field of the Invention
The present invention relates to a group key update which updates a key of members in a group. More particularly, the present invention relates to a rekey index generation method and a rekey index generation apparatus which allows a member in a group to effectively select a necessary encoded key from among encoded keys transmitted from a server.
2. Description of Related Art
Contents which are provided to group members are typically encrypted, so that other users can not use the contents. Accordingly, all of the group members should have an encryption key, provided from a server, capable of decoding the encrypted contents.
In this case, updating an encryption key is an important issue. For example, when new members join a group, the group is required to allow the new member to restrictively access contents. Later, after the new member joins the group, when the existing group members update the encryption key, and the new member may share the updated key.
Also, when one of the existing group members leaves the group, the group key, used by all group members before the leaving member leaves the group, is required to be updated so that the leaving member no longer has access to the contents.
Generally, methods of updating the group key are performed in two ways.
In one method, a server calculates an updated key and transmits the updated key to a member requiring the updated key, when it is required to update a group key. This method is problematic due to the great burden placed upon the server since the server is required to calculate the group key for all group members who require the updated group key, and to transmit the calculated group key.
In another method, members that are capable and require the update voluntarily calculate the group key and perform a required update, while the server transmits the updated key to only those members incapable of performing a self-update when it is required to update the group key. In the case of this method, of the burden placed upon the server is not great since the server calculates to transmit the updated group key to only those members incapable of performing the self-update.
A member device that downloads an updated key from a server is required to select necessary information for the member device from among updated keys transmitted from the server. For this, the server transmits a rekey index, and the member device receives the transmitted rekey index and selects a necessary key from among the received updated keys, using information inserted into the rekey index.
Accordingly, there is a need for an improved rekey index generation method and rekey index generation apparatus in order to effectively select a necessary update key which is required in a device.